This invention relates to a method of extracting hexavalent chrome capable of reliably and cost-efficiently extracting hazardous hexavalent chrome remaining on the surface or inside of chrome plating or chromate film without a need of a special and expensive equipment, preventing elution of the hexavalent chrome from a raw material to be processed, enjoying safe use and discarding of the part where the hexavalent chrome remains, and preventing environmental pollution and adverse effect to a human body.
Various surface processing such as plating and chemical treatment are applied to a raw material such as iron, nonferrous metal and synthetic resin for the purposes of rust-proof, decoration, modification and the like. Among them, chrome plating is widely adopted in the field of automobiles, electronics and electric equipments as decorative chrome plating or industrial chrome plating because the chrome plating is excellent in decorative property, corrosion resistance and wear resistance.
The chrome plating bath generally requires a large amount of chromic acid, i.e., hexagonal chrome as a supplying source of chromic ions. Therefore, a careful countermeasure is needed for safety, sanitation, pollution, etc. Moreover, it is pointed out that hexavalent chrome remaining in the deposited metallic chrome and chrome oxide is eluted by rain water or the like to pollute the underground water and environment and gives a significant adverse effect to a human body.
In view of the above, legal restriction is studied in the Euro-American automobile industry, for reducing the consuming amount of hexavalent chrome and for prohibiting the use of the same. Similar movements can be seen in the electronic and electric equipment industries. Movements following thereto can also be seen in Japan.
Moreover, a chromating processing as the above-mentioned chemical treatment is used as a post-treatment of zinc plating products and cadmium plating products. In such processing, a workpiece as a raw material to be processed is immersed in an elution which chiefly contains chrome acid or heavy chrome acid, so that an anticorrosive film as a thin passivated film is generated on the surface of the workpiece. Here again, it is pointed out that the hexavalent chrome remaining on the surface of the workpiece is eluted by rain water which results in environmental pollution and gives significant adverse effect to a human body.
Thus, it is urgently required to develop technique which can take the place of the hexavalent chrome or chromating processing. The main stream of the development is a trivalent chrome basis. There are many in which a film of trivalent chrome is top-coated. Although those alternative techniques are capable of producing, at a test level, products equivalent to the hexavalent or chromating processing, there still remains a problem at a mass production level, and no sufficient products are obtained yet.
On the other hand, the existing chrome plated or chromate-processed parts have the afore-mentioned problem of environmental pollution caused by elusion of hexavalent chrome and so, those contaminate parts cannot be discarded directly. Moreover, since use of the existing parts is not reliable, it is urgently required to solve those problems.
Particularly, WEEE instructions and RoHS instructions pertaining to the limitation of use of hazardous substances contained in electronic and electronic equipments are expected to come to be effective in near future in Euro-countries. Like restrictions are also expected to come in force in Japan. Thus, a countermeasure is urgently required.
As solving means for coping with those problems, there are some methods of extracting or removing hexavalent chrome.
For example, there is known a method which includes a reaction vessel for adding a reducing agent to a factory effluent therein and a precipitation vessel for generating chrome precipitates by adding a precipitant to the effluent introduced from the reaction vessel, measuring means for measuring the hexavalent chrome concentration in the effluent being disposed at a pipeline for connecting those vessels and an effluent feed pipeline, the amount of the reducing agent to be added being controlled based on the value measured by the measuring means, blocking of the pipeline caused by the chrome precipitates being prevented by adjusting the amount of the chrome precipitates, thereby efficiently removing the chrome remaining in the effluent (for example, see Japanese Patent 1).
However, since the above-mentioned chrome removing method is designed for removing the hexavalent chrome in the effluent, it is unable to cope with the requirements for removing the hexavalent chrome remaining in a solid workpiece. Moreover, the equipment necessary for carrying out this method is expensive because it requires a reaction vessel, a precipitation vessel. measuring means and the like. In addition, a space for installing those component parts is large.
As a method capable of solving those problems, there is known a method for processing a hexavalent chrome-containing waste including a mixing step for making a mixture by adding and mixing a reducing agent to a hexavalent chrome-containing waste such as magnesia-chrome brick and chrome-magnesia brick containing Mg(II) and Cr(III), and hazardous Cr (IV), a reducing step for reducing Mg(II) to a metallic magnesium by heating the mixture in an oxygen non-containing atmosphere and reducing and non-polluting Cr (IV), and a separating and collecting step for separating and collecting a metallic magnesium from the reduced mixture by evaporating the metallic magnesium simultaneously with the reducing process (see Japanese Patent 2).
In the above-mentioned processing method, the magnesia-chrome brick is not used as it is. The method requires a step for pulverizing the magnesia-chrome brick into fine powders as well as a pulverizing equipment. Thus, much time and labor are required for processing the waste and in addition, a complicate and troublesome step is required. Thus, productivity is poor.    Japanese Patent 1: Japanese Patent Application Laid-Open No. 2005-87988.    Japanese Patent 2: Japanese Patent Application Laid-Open No. H11-169814.
It is an object of the present invention to provide, in order to solve the above-mentioned problems, a hexavalent chrome extracting method capable of reliably and cost-efficiently extracting hazardous hexavalent chrome remaining on the surface or inside of chrome plating or chromate film without a need of a special and expensive equipment, preventing elution of the hexavalent chrome from a raw material to be processed, enjoying safe use and discarding of a part where the hexavalent chrome remains, and preventing environmental pollution and adverse effect to a human body.